Pressurized refrigerant recirculation system with control means

ABSTRACT

A refrigeration apparatus having an accumulator-separator with a pair of pumping tanks which are sequentially operated by high pressure gas from the receiver for pumping liquid refrigerant to an evaporator. The apparatus provides a foolproof fail-safe automatically operated system in which the accumulator-separator and the pumping tanks are of a size to accommodate all of the refrigerant within the system so that a rise in pressure induced by temperature increase caused by a power failure is not sufficient to rupture the system.

United tates tent [1 1 [1111 3,827,249 Garland et al, [45] A 6, 1974 PMIESSURIZED REFRIGERANT 2,952,137 9/1960 Watkins...... 62/174 RECIRCULATION SYSTEM WITH 3,643,460 2/1972 Garland 62/512 CONTROL MEANS P n E M P l rzmary xammer eyer er in [75] Inventors: Milton W. Garland, Waynesboro, A A t Yat D n J Pa.; Robert 6. Fish, St. Louis, Mo. Omey gen m es Owe [73] Assignee: Frick Company, Waynesboro, Pa. [57] ABStCT [22] Filed: Mar, 12, 11973 A refrigeration apparatus having an accumulatorseparator with a pair of pumping tanks which are se- [211 App! 340638 quentially operated by high pressure gas from the receiver for pumping liquid refrigerant to an evaporator. [52] US. Cl 62/174, 62/218, 62/470, The apparatus provides a foolproof fail-safe automati- 62/512 cally operated system in which the accumulator- [51] Int. Cl. F25!) 43/00 separator a th pump ng tanks are f a size to ac- [58] Field at Search 62/174, 512 Commedate all f the r g rant ithin the system so i that a rise in pressure induced by temperature in- [56] References Cited crease caused by a power failure is not sufficient to UNITED STATES PATENTS rupture the System 2,871,673 2/1959 Richards 62/174 7 Claims, 4 Drawing Figures CONDENSER 2| PATENTEB AUG 6 I974 SHEU 2 IF 3 mmmzmozoo PRESSURIZED REFRIGERANT RECIRCULATION SYSTEM WITH CONTROL MEANS BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates generally to refrigeration apparatus and relates particularly to a pressurized refrigerant recirculation apparatus which recirculates liquid refrigerant through an evaporator by means of high pressure gas from the receiver.

2. Description of the Prior Art l-leretofore many efforts have been made to recirculate pressurized refrigerant through relatively large evaporators such as those used in ice rinks and the like and these have included systems having an accumulator connected to an auxiliary accumulator or reservoir which collects liquid refrigerant from a flooded evaporator as well as from a compressor and condensor and pumps such liquid refrigerant to an evaporator. Some examples of prior art which teach this general concept are the US. Pats. to Gay, No. 1,994,037; Phillips, No. 2,570,979; Watkins, No. 2,590,741; Christensen, No. 2,778,195; Watkins, No. 3,164,973; Grant, No. 3,214,932; and Garland et al., No. 3,353,367.

Some efforts have been made to provide a pair of pumping tanks for forcing liquid refrigerant into an evaporator by alternately receiving liquid refrigerant from an accumulator into one tank while pressurizing the other pumping tank with high pressure liquid refrigerant. An example of this type of structure is the US. Pat. to Watkins, No. 2,952,137.

SUMMARY OF THE INVENTION The present invention is a fail-safe reliable automatically operated refrigerant recirculation system having an accumulator-separator communicating with a pair of alternately operable pumping tanks which are operated sequentially in timed relationship, or are operated in accordance with the amount of liquid refrigerant within the accumulator as indicated by a float control. While one of the pumping tanks is receiving liquid refrigerant under low pressure conditions existing within the accumulator-separator, the other pumping tank is pressurized by high pressure gas from the receiver to force liquid refrigerant from such pumping tank to the evaporator. In the event of a power failure, both of the pumping tanks automatically communicate with the accumulator-separator and are of a size to accommodate all of the refrigerant within the system so that a rise in pressure induced by a temperature increase of the refrigerant is not sufficient to rupture the system.

It is an object of the invention to provide a pressurized refrigerant recirculation system having an accumulator-separator and a pair of pumping tanks in which the pumping tanks are automatically sequentially pressurized by high pressure gas from the upper portion of the receiver so that one of such tanks is supplying liquid refrigerant under pressure to an evaporator, while the other tank is receiving liquid refrigerant from the accumulator-separator.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic layout of a recirculation system operated in accordance with the quantity of refrigerant charge in the system.

FIG. 2 is a schematic layout similar to FIG. 1 in which the system is operated in. accordance with the amount of liquid refrigerant within the accumulator-separator.

FIG. 3 is an electrical diagram of a time operated control system.

FIG. 4 is an electrical diagram of a float operated control system.

DESCRIPTION OF THE PREFERRED EMBODIMENTS With reference to FIG. 1, a relatively large refrigeration system is provided having an evaporator which discharges a mixture of liquid and gaseous refrigerant to an accumulator-separator 11 through a discharge line 12. Gaseous refrigerant is withdrawn from the accumulator-separator through conduits l3 and 14 by the suction side of compressors 15 and 16, respectively. These compressors compress and discharge refrigerant through oil separators 117 and 18 and through conduits l9 and 20, respectively, into a condenser 21. Liquid refrigerant from the condenser is discharged through a conduit 22 into a receiver 23.

Within the receiver the liquid level is maintained at a predetermined level by a float operated valve 24. A normally closed pressure operated flow control valve 25 is connected by a conduit 26 to the float valve 24,

and a conduit 27 connects the float valve structure to the accumulator-separator 11. When the liquid level in the receiver 23 raises the float valve shutoff stem from its seat, the conduits 26 and 27 relieve the pressure within the valve 25 so that liquid under pressure within the receiver is discharged through a discharge line 28 into the valve 25 to cause the valve to open so that the liquid refrigerant flows through an inlet line 29 into the accumulator-separator 11.

Liquid refrigerant from the accumulator-separator drains by gravity through a conduit 32 into a header 33 and through a pair of conduits 34 and 35 having check valves 36 and 37 into a pair of pumping tanks 38 and 39, respectively. The pumping tank 38 is connected by a conduit 40 to a normally open pressure operated vent valve 41 which communicates with the accumulatorseparator 11 through a conduit 42. The pumping tank 39 is connected by a conduit 43 to a normally open pressure operated vent valve 44 which communicates with the accumulator-separator by a conduit 45 and the conduit 27. Under normal shutdown or no-power conditions, the vent valves 41 and 44 both are open and permit liquid levels to equalize within the pumping tanks 38 and 39.

The vent valve 41 may be selectively closed by connecting the upper portion thereof to a pressure line 46 communicating with the upper portion of the receiver 23 to permit pressurized gas to flow through such line under the control of an in-line gate valve 47 selectively operated by a solenoid 48. The vent valve 44 may be selectively closed by connecting the upper portion to a pressure line 49 connected to a pressure line 46. Flow of gas under pressure through the pressure line 49 is controlled by an in-line gate valve 50 selectively operated by a solenoid 51.

In order to selectively pressurize the pumping tanks 38 and 39, a tank pressurizing line having an in-line pressure regulating valve 56 is connected to the pressure line 46 which leads into the upper portion of the receiver 23 and the opposite end of the line 55 is connected to one side of a normally closed pressure operated flow control valve 57. The opposite side of such flow control valve is connected by a pressurizing line 58 to the conduit leading into the pumping tank 38 when the vent valve 41 is closed. A branch line 59 connects the tank pressurizing line to one side of a normally closed pressure operated flow control valve 60 and the opposite side of such valve is connected by a pressurizing line 61 to the conduit 45 which leads into the pumping tank 39 when the vent valve 44 is closed.

The pressure operated valve 57 is connected by a vent line 62 to the accumulator-separator 11 and such vent line has an in-line gate valve 63 operated by a solenoid 64 to permit selective operation of the valve 57. The pressure operated valve 60 communicates with the accumulator-separator 11 through a vent line 65 having an in-line gate valve 66 selectively operated by a solenoid 67.

The pressurizing of the pump tank 38 causes liquid refrigerant therein to be discharged through the conduit 34 and through an outlet conduit 68 having a check valve 69 into a discharge conduit 70 from which the pressurized liquid refrigerant flows through a strainer-drier 71 and through a conduit 72 to one side of a normally closed pressure operated flow control valve73.

In order to open the normally closed valve 73, a vent line is provided in communication with the accumulator-separator 11 and such vent line has an inline gate valve 81 selectively operated by a solenoid 82. The opposite side of the flow control valve 73 is connected by a conduit 74 to the evaporator 10 with the direction of flow being controlled by a check valve 75.

When the liquid refrigerant in the pumping tank 38 is substantially exhausted, such pumping tank is vented automatically and the pumping tank 39 is pressurized by closing the normally open vent valve 44 and opening the flow valve 60. When the tank 39 is pressurized, liquid refrigerant therein is forced through conduit 35 into an outlet conduit 76 having a check valve 77, into the discharge conduit 70 and then to the evaporator 10.

Refrigerant 22 (chlorodifluoromethane) normally is used in the present system and such refrigerant is miscible with oil from the compressors 15 and 16 and therefore an oil still 83 is provided having a heater 84 for separating liquid refrigerant from oil. When either of the pumping tanks 38 or 39 is pressurized, a flow of refrigerant and oil passes through conduits 85 and 86 having throttling valves 87 and 88 respectively, and through check valves 89 and 90 to an inlet conduit 91 communicating with the oil still 8. Liquid refrigerant which has been separated within the oil still is discharged through a conduit 92 and returned to the accumulator-separator 11. The conduit 92 has a pressure regulating valve 93 to maintain the pressure within the oil still at a level less than the pressure within the pumping tanks 38 and 39 but greater than the pressure within the accumulator-separator l1, and above the crank case pressure of the compressors 15 and 16.

Oil which is separated from the refrigerant within the still 83 is discharged through conduits 94 having an inline gate valve 95 controlled by a solenoid 96. The solenoid 96 is controlled by a thermostatic switch 97 within the oil still 83. From the conduit 94, oil from the still 83 flows through a conduit 98 connected to oil level devices 99 and 100 associated with the compressors l5 and 16, respectively.

Although the vent valves 41 and 44 and the flow control valves 25, 57, 60 and 73 have been illustrated and described as pressure operated valves, it is apparent that other conventional valves could be used.

The structure thus far described with reference to FIG. 1 discloses a system in which the level of the liquid refrigerant within the accumulator-separator 11 is variable with the amount of refrigerant charge in the system. The amount of the refrigerant charge varies in accordance with the specific refrigerant recirculation system. As an example, an ice rink of approximately 85 feet wide by 200 feet long normally requires approximately 1,000 gallons or 7,000 pounds of refrigerant 22. Under a normal shutdown or no-power condition, the pressure operated valves 25, 41, 44, 57, 60 and 73 return to their normal operating position illustrated in FIG. 1. Under these conditions, both of the pumping tanks 38 and 39 communicate with the accumulatorseparator 11 and such pumping tanks and accumulatorseparator are of a size to accommodate all of the refrigerant within the system so that a rise in pressure induced by a temperature increase of the refrigerant is not sufficient to rupture the system.

With reference to FIG. 2, a modified refrigerant recirculation system is disclosed in which the level of liquid refrigerant within the accumulator-separator 11 is maintained by a float operated valve 101. In this modification, the float valve 101 is responsive to the liquid level within the accumulator-separator so that when the liquid refrigerant therein drains into one of the pumping tanks 38 and 39, the float valve shutoff stem rises from its seat and permits liquid refrigerant from a relatively large receiver 23 to flow through a conduit 102 into the accumulator-separator and replenish the supply therein. An in-line gate valve 103 operated by a solenoid 104 permits liquid refrigerant to flow through the conduit 102 when the system is in operation but closes such conduit when the system shuts down.

With reference to FIGS. 3 and 4, either of the modifications illustrated in FIGS. 1 and 2 can be controlled by a program timer, as illustrated in FIG. 3, or either of the modifications can be operated by float level switches, as illustrated in FIG. 4.

With particular reference to FIG. 3, all of the connections above the dotted-line A-A are conventional for automatic compressor operation in refrigeration systems and therefore a detailed description is not be lieved to be necessary. In the area below the dotted line AA, the lines have been numbered for clarity and the operation of the device will be described in conjunction with the description of FIG. 3.

To begin operation, a manual switch 105 (line 3) is moved on the On position to energize a program motor 106 (line 1) after the compressors have been started providing the compressor inlet pressure is below a level which will not overload the compressor driving motor. A pressure limiting switch 107 (line 3) provides the protection for the compressor driving motor. When the pressure limiting switch 107 and the manual switch 105 are both closed, the program motor 106 is started, and an indicating signal such as a green light 108 (line 2) and a control relay 109 (line 3) both are energized. A cam 110 having lobes 111 and 112 is attached to the output shaft of the program motor 106. The lobe 111 operates switches 113 (line 4) and 114 (line 7) and lobe 112 operates switches 115 (line 6) and 116 (line 9).

When the control relay 109 is energized, control relay switches CRS (lines 4, 6, 10 and 12) are closed. In this position, closed switch 114 (line 7) energizes solenoid 51 to open the gate valve 50 and permit refrigerant gas under pressure from the receiver 23 to close the vent valve 44 so that the pumping tank 39 is no longer vented to the accumulator-separator 11. Also, switch 114 energizes a red indicator light or other signal 117 (line 8) to indicate that pumping tank 39 is being pressurized. Simultaneously closed switch 116 (line 9) energizes solenoid 67 to open the gate valve 66 and relieve the pressure within the flow valve 60 so that such valve opens.

Gas pressure from the receiver 23 is constantly regulated by the pressure relief valve 56 as such gas passes through the pressurizing line 55, branch line 59, flow valve 60 and pressurizing line 61 into the pumping tank 39 to pressurize such tank and force the liquid refrigerant therein through the outlet conduit 76 into the discharge conduit 70, strainer-drier 71 and conduit 72 into the flow valve 73. When the control relay 109 was energized, solenoid 82 (line 10) was energized to open gate valve 81 so that introduction of liquid refrigerant under pressure into the flow valve 73 opens such valve and permits pressurized liquid refrigerant to flow through the conduit 74 into the evaporator 10.

As the program motor 106 rotates the cam 110, such cam causes switches 114 (line 7) and 116 (line 9) to open and simultaneously causes switches 113 (line 4) and 115 (line 6) to close. The opening of switches 114 and 116 deenergizes solenoids 51 and 67 so that the introduction of gas under pressure into the pumping tank 39 is interrupted by the closing of flow valve 60, and the opening of vent valve 44 vents the pumping tank 39 to the accumulator-separator 11. This permits liquid refrigerant within the accumulator-separator to drain through the conduit 32, header 33 and conduit 35 into the pumping tank 39.

The closing of switches 113 (line 4) and 115 (line 6) energizes solenoids 48 and 64, respectively, so that the vent valve 41 closes and flow valve 57 opens to permit refrigerant gas under pressure to flow through the pressurizing line 55, valve 57 and pressurzing line 58 into the pumping tank 38. Simultaneously, the closing of switch 113 (line 4) energizes a red indicator light or other signal 118 (line 5) to indicate that pumping tank 38 is being pressurized. The gas under pressure forces liquid refrigerant from the pumping tank 38 through the outlet conduit 68 into the discharge conduit 70, strainer-drier 71, conduit 72, flow valve 73 and conduit 74 into the evaporator.

Alternate pressurizing of the pumping tanks 38 and 39 permits liquid refrigerant to flow under low accumulator-separator pressure into one of such pumping tanks while the other pumping tank is discharging liquid refrigerant to the evaporator. This cycle is continuously repeated until the refrigeration system is shut off by any means of control, such as thermostat, pressurestat, oil failure switch, manual shutdown, power failure, or other means at which time the program motor 106 stops. The absence of a completed circuit causes all valves to return to their normal position as indicated in FIGS. 1 and 2. In this position, both of the pumping tanks 38 and 39 are in non-pressurized condition so that liquid refrigerant from the accumulator-separator 11 drains into both pumping tanks.

The electrical wiring diagram shown in full lines in FIG. 3 is for use with a refrigeration system as shown in FIG. 1 under program time control. If the system shown in FIG. 2 is used with a program time control, an additional connection for the solenoid 104 of gate valve 103 is added and such connection is shown by broken lines (line 11).

Instead of the time program control previously described, each of the refrigeration systems shown in FIGS. 1 and 2 can be operated in accordance with the level of the liquid refrigerant within the pumping tanks 38 and 39. In order to do this, a float control device 120 is added to the pumping tank 138 and a float control device 121 is added to the pumping tank 139. The float control devices 120 and 121 are shown in broken lines in FIGS. 1 and 2.

With particular reference to FIG. 4, a wiring diagram is shown for controlling the recirculation systems of FIGS. 1 and 2 when using float control devices 120 and 121. As indicated in FIG. 3, all connections located above the dotted line A-A of FIG. 4 are conventional for automatic compressor operation in refrigeration systems.

At the beginning of operation, manual switch 105 (line 2) is moved to the On position to energize control relay 109 (line 2) and the green light 108 (line 1), if the compressors 15 are running and the pressure is down to the level where pressure limiting switch 107 is closed. Energizing the control relay 109 closes all switches CRS (lines 4, 6, 10 and 1.4). At the same time, a timer 122 (line 3) is energized and after a delay of approximately 30 seconds the timer 122 closes timer switch 123 (line 10) and remains closed until the timer 122 is deenergized. The 30 second delay prevents the energizing of a control relay 124 (line 11) and permits a control relay 125 (line 7) to energize and demobilize control relay 124 (line 11) by opening control relay switch 126 (line 10).

Operation of control relay 125 closes normally open control relay switches 127 and 128 to operate solenoid 48 to open gate valve 47 and close the vent valve 41 and simultaneously operates solenoid 64 to open gate valve 63 so that high pressure from the pressurizing line 55 passes through the pressure operated flow valve 57 and introduces fluid under pressure into the pumping tank 38 to cause the liquid refrigerant therein to be discharged to the evaporator. A red indicating light 129 (line 8) is energized during the time of pressurization of the pumping tank 38.

When the liquid level in pumping tank 38 falls below the float control device 120, float switch 130 (line 6) opens and deenergizes control relay 125 (line 7). When control relay 125 is deenergized, switches 127 (line 7) and 128 (line 9) open and switch 126 (line 10) closes to energize control relay 124 (line 11). Closing of the control relay 124 opens normally closed control relay switch 131 (line 6) and closes normally open relay switches 132 and 133 (lines 11 and 13, respectively).

Closing of the control relay switch 132 energizes sole:

noid 51 to open gate valve 50 and introduce fluid under pressure into the vent valve 44 and cause such valve to close. Simultaneously control relay switch 133 (line 13) energizes solenoid 67 to open the gate valve 66 and permit fluid under pressure from the pressurizing line 55 to open pressure operated valve flow 60 and introduce fluid under pressure into the pumping tank 39. A red indicating light 135 (line 12) is energized during the time of pressurization of the pumping tank 39.

Pressurizing of the pumping tank 39 discharges liquid refrigerant from the tank until the liquid level falls below the float control device 121 so that float switch 134 (line 10) opens and deenergizes control relay 124 (line 11). The deenergizing of control relay 124 closes control relay switch 131 (line 6) to energize control relay 125 (line 7), if sufficient liquid refrigerant from the accumulator-separator 11 has drained into the pumping tank 38 to a level high enough to close float switch 130 (line 6). Simultaneously control relay switches 132 and 133 (lines 11 and 13) are returned to normally open position to deenergize solenoids 51 and 67, respectively, to depressurize pumping tank 39 so that liquid refrigerant from the accumulator-separator drains into such pumping tank. These cycles continue as long as any compressor is in operation.

Compressor shutdown from power failure or any other cause discontinues the cycling process and causes all control valves to assume the normally open or closed position as indicated in FIGS. 1 and 2.

The electrical connections shown in solid lines in FIG. 4 are for the program time control system illustrated in FIG. 1. If the float control system illustrated in FIG. 2 is used, solenoid 104 (line 5) shown in broken lines, is provided for operating gate valve 103 to permit liquid refrigerant to flow from the receiver 23 through conduit 102 into the accumulator-separator 11 in response to the operation of float valve 101.

We claim:

1. A safe automatically operated refrigerant recirculation apparatus for circulating liquid refrigerant through an evaporator and usable with conventional compressor means and condenser means, said apparatus comprising an accumulator-separator for receiving liquid and gaseous refrigerant from the evaporator and separating liquid refrigerant from gaseous refrigerant, conduit means connecting said accumulator-separator with said compressor means for removing gaseous refrigerant from said accumulator-separator, said compressor means compressing said gaseous refrigerant and discharging the same to said condenser means, a receiver communicating with said condenser for receiving liquid and gaseous refrigerant therefrom under condenser pressure, means for discharging liquid refrigerant from said receiver into said accumulatorseparator, a pair of pumping tanks connected to said accumulator-separator and adapted to receive liquid refrigerant therefrom under pressure conditions existing in the accumulator-separator, means for sequentially introducing high pressure gaseous refrigerant only from the upper portion of said receiver into said pumping tanks, said last mentioned means including a pressurizing conduit connecting the upper portion of said receiver to said pumping tanks, selectively operated flow valve means in said pressurizing conduit for each of said pumping tanks, selectively operated vent valve means associated with each of said pumping tanks, and control means for automatically and sequentially operating said flow valves and said vent valves, whereby the vent valve of one of said tanks is closed when the flow valve of said one tank is open to introduce gaseous refrigerant under pressure into said one pumping tank and cause liquid refrigerant to be discharged therefrom into the evaporator while the vent valve of the other pumping tank is open and flow valve is closed to permit liquid refrigerant to flow from said accumulator-separator into said other pumping tank.

2. The structure of claim 1 in which each of said vent valve means is normally open and each of said flow valve means is normally closed so that refrigerant flows from said accumulator-separator into both of said pumping tanks when the compressor means is not operating.

3. The structure of claim 1 in which said vent valve means and said flow valve means are pressure operated.

4. The structure of claim 1 including an oil still communicating with each of said pumping tanks, said oil still separating refrigerant from oil entrained therein, means for returning refrigerant from said still to said accumulator-separator, and means for returning oil from said still to said compressor means.

5. The structure of claim 1 in which the level of liquid refrigerant within said receiver is controlled by float operated valve means.

6. The structure of claim 1 in which the level of liquid refrigerant within said accumulator-separator is controlled by float operated valve means.

7. In a refrigerant recirculation system having an evaporator, an accumulator-separator, at least one compressor receiving gaseous refrigerant from said accumulator-separator and discharging the same to a condenser, a receiver receiving refrigerant from said condenser, said receiver containing both liquid and gaseous refrigerant under condenser pressure, means for discharging liquid refrigerant from said receiver into said accumulator-separator, and a pair of pumping tanks for receiving liquid refrigerant from said accumulator-separator, the improvement comprising vent valve means associated with each of said pumping tanks for selectively relieving pressure therein, flow valve means associated with each of said pumping tanks for selectively introducing gaseous refrigerant under pressure into said tanks, each of said flow valve means communicating with the upper portion of said receiver for introducing gaseous refrigerant only under condenser pressure into said pumping tanks, and control means for selectively operating said vent valve means and said flow valve means, whereby gaseous refrigerant from said receiver is introduced into one of said pumping tanks to discharge liquid refrigerant therefrom to said evaporator while liquid refrigerant from said accumulator-separator is being discharged into the other of said pumping tanks. 

1. A safe automatically operated refrigerant recirculation apparatus for circulating liquid refrigerant through an evaporator and usable with conventional compressor means and condenser means, said apparatus comprising an accumulatorseparator for receiving liquid and gaseous refrigerant from the evaporator and separating liquid refrigerant from gaseous refrigerant, conduit means connecting said accumulator-separator with said compressor means for removing gaseous refrigerant from said accumulator-separator, said compressor means compressing said gaseous refrigerant and discharging the same to said condenser means, a receiver communicating with said condenser for receiving liquid and gaseous refrigerant therefrom under condenser pressure, means for discharging liquid refrigerant from said receiver into said accumulator-separator, a pair of pumping tanks connected to said accumulator-separator and adapted to receive liquid refrigerant therefrom under pressurE conditions existing in the accumulator-separator, means for sequentially introducing high pressure gaseous refrigerant only from the upper portion of said receiver into said pumping tanks, said last mentioned means including a pressurizing conduit connecting the upper portion of said receiver to said pumping tanks, selectively operated flow valve means in said pressurizing conduit for each of said pumping tanks, selectively operated vent valve means associated with each of said pumping tanks, and control means for automatically and sequentially operating said flow valves and said vent valves, whereby the vent valve of one of said tanks is closed when the flow valve of said one tank is open to introduce gaseous refrigerant under pressure into said one pumping tank and cause liquid refrigerant to be discharged therefrom into the evaporator while the vent valve of the other pumping tank is open and flow valve is closed to permit liquid refrigerant to flow from said accumulator-separator into said other pumping tank.
 2. The structure of claim 1 in which each of said vent valve means is normally open and each of said flow valve means is normally closed so that refrigerant flows from said accumulator-separator into both of said pumping tanks when the compressor means is not operating.
 3. The structure of claim 1 in which said vent valve means and said flow valve means are pressure operated.
 4. The structure of claim 1 including an oil still communicating with each of said pumping tanks, said oil still separating refrigerant from oil entrained therein, means for returning refrigerant from said still to said accumulator-separator, and means for returning oil from said still to said compressor means.
 5. The structure of claim 1 in which the level of liquid refrigerant within said receiver is controlled by float operated valve means.
 6. The structure of claim 1 in which the level of liquid refrigerant within said accumulator-separator is controlled by float operated valve means.
 7. In a refrigerant recirculation system having an evaporator, an accumulator-separator, at least one compressor receiving gaseous refrigerant from said accumulator-separator and discharging the same to a condenser, a receiver receiving refrigerant from said condenser, said receiver containing both liquid and gaseous refrigerant under condenser pressure, means for discharging liquid refrigerant from said receiver into said accumulator-separator, and a pair of pumping tanks for receiving liquid refrigerant from said accumulator-separator, the improvement comprising vent valve means associated with each of said pumping tanks for selectively relieving pressure therein, flow valve means associated with each of said pumping tanks for selectively introducing gaseous refrigerant under pressure into said tanks, each of said flow valve means communicating with the upper portion of said receiver for introducing gaseous refrigerant only under condenser pressure into said pumping tanks, and control means for selectively operating said vent valve means and said flow valve means, whereby gaseous refrigerant from said receiver is introduced into one of said pumping tanks to discharge liquid refrigerant therefrom to said evaporator while liquid refrigerant from said accumulator-separator is being discharged into the other of said pumping tanks. 